1. Field of the Invention
The present invention relates to a display device, and more particularly to an organic light emitting diode display having shield electrodes.
2. Description of the Prior Art
The polymeric or organic light emitting diodes (OLEDS) are electroluminescent(EL) layers that emit light generated by radiative recombination of injected electrons and holes within one or more organic EL layers or an organic EL region thereof.
The OLED displays employing the OLED have been attracting more attention because of their various advantages including simple structures, fast response times and wide viewing angles.
FIG. 1A is a top view of a general active matrix OLED(AMOLED) display device. In FIG. 1A, a reference numeral 10 denotes an OLED display device. The OLED display device 10 has an OLED display 12, a gate multiplexer 16, and a driver 18. The OLED display 12 consists of a plurality of pixels 14 in a matrix array form for displaying images, for example. A reference numeral C denotes a portion of the OLED display 12 in store for enlargement of pixels
FIG. 1B is a partially enlarged view of the OLED display of FIG. 1A, taken from the portion C of FIG. 1A. As shown in FIG. 1B, each of the pixels 14 has an OLED 22 and a thin-film (TFT) drive circuit 24. Address and data lines in FIG. 1B are connected to the gate multiplexer 16 and the driver 18, respectively, as shown in FIG. 1A.
FIG. 1C is a cross-sectioned view for one pixel, taken along lines A–A′ of FIG. 1B, showing that the OLED 22 and the TFT drive circuit 24 are formed on a glass substrate 32.
The substrate 32 may be transparent or opaque. Thus, the OLED display may be configured to emit light through the substrate 32 or through the cathode layer. FIG. 1C shows such a pixel emitting light through the substrate 32.
In the OLED 22, the anode layer is typically made of a transparent conducting material, while the cathode layer is typically made of a conducting metal with a low work function.
The anode layer is formed as a transparent bottom electrode, while the cathode layer is formed as a continuous top electrode over the OLED layer.
In a general AMOLED display, it is important to ensure that the aperture ratio or fill factor, which is defined as the ratio of light emitting display area to the total pixel area, should be high enough to ensure display quality.
The AMOLED display 10 in FIG. 1A is based on light emission through an aperture on the glass substrate 32 where the backplane electronics is integrated. Increasing the on-pixel density of TFT integration for stable drive current reduces the size of the aperture. The same problem happens when pixel sizes are scaled down.
The solution to having an aperture ratio that is invariant on scaling or on-pixel integration density is to vertically stack the OLED layer on the backplane electronics or the TFT drive circuit, along with a transparent top electrode. FIG. 2 is a cross-sectioned view for a pixel formed with an OLED vertically stacked on the backplane electronics. In FIG. 2, a reference number 14 denotes the pixel. The pixel 14 has the OLED 31 and the backplane electronics 24 which are vertically stacked on the substrate 32. Reference numerals T1 and T2 denote thin-film transistors respectively. A more description of FIG. 2 can be found, later, when FIG. 3 is described.
The operations of the AMOLED display device 10 having the above structure will be described in detail with reference to FIGS. 1A–1D.
FIG. 1D is a view for showing an equivalent circuit of FIG. 1C. A 2-transistor driver circuit realized, for example, in polysilicon technology is illustrated for the equivalent circuit. The equivalent circuit can be applied to the AMLOED display as shown in FIG. 2 using amorphous silicon technology but with variation in TFT type and OLED location.
When a voltage Vaddress from the gate multiplexer 16 activates one of the address lines, a thin-film transistor T1 is turned on so that a voltage Vdata from the driver 18 starts charging a capacitor Cs through one of the data lines. At this time, the voltage Vdata also causes a gate capacitance of a driver thin-film transistor T2. Depending on the voltage Vdata on the data line, the capacitor Cs charges up to turn the driver transistor T2 on, which then starts conducting to drive the OLED 22 with an appropriate level of current. When the address line is turned off, the transistor T1 is turned off, and a voltage at the gate of the driver transistor T2 remains almost the same. Hence, the current through the OLED remains unchanged after the turn-off of the transistor T1. The current of the OLED changes only the next time around when a different voltage is written into the pixel.
However, the continuous back electrode can give rise to parasitic capacitance causing the leakage current of the transistor T1, whose effects can become significant enough to affect the operation of the driver transistor T2 when the continuous back electrode runs over the switching and other thin film transistors. That is, the presence of the continuous back electrode can induce a parasitic channel in thin-film transistors giving rise to high leakage current. The leakage current is the current that flows between the source and drain of the thin-film transistor T1 when the gate of the thin-film transistor T1 is in its OFF state.
The leakage current could drain away the charge on the gate of the driver thin-film transistor T2 by discharging the capacitance that keeps it continuously in its ON state. Accordingly, it adds to the total power consumption of the display.